Apparently the drain pipes under the main bar have turned to dust. The reason the floor sinks weren't draining properly is that they were draining directly into the sand and burnt timber upon which our building rests.

These aren't even ancient pipes: they date back only to 2001, when we installed the main bar against the front wall. But the plumber we had back then was a complete crook, so I'm sure he screwed us by cheaping out on it in some way or another.